During cement clinker production, mercury is emitted into the environment, primarily in the gaseous state. Elemental mercury and mercury (II) chloride are the dominant species. The latter is water-soluble and becomes highly concentrated in foodstuffs, in particular in fish. It is then absorbed by humans via food. Owing to its high level of toxicity, considerable health risks are connected therewith. The regulatory authorities therefore impose on cement manufacturers increasingly strict threshold values for the permissible emissions. The permissible emission threshold values are also decreasing for other materials which are harmful to humans or the environment, such as dioxins (PCDD), furans (PCDF), long-chained hydrocarbon compounds (VOC) and nitrogen oxides (NOx).
A known method for reducing mercury emissions is described in U.S. Pat. No. 5,505,766 A1 and known under the name TOXECON. In this instance, a highly porous particulate adsorbent (which is referred to as activated carbon below) which comprises primarily carbon is injected at a low concentration into an exhaust gas flow which contains mercury. The mercury is deposited on the activated carbon particles and is then separated in a filter. In the production process for cement, it is disadvantageous for the exhaust gas flow to also contain exhaust gas dust from the combustion or grinding process, which dust is composed primarily of calcium, iron, aluminium and silicon dioxides or carbonates and has a mass flow which is several orders of magnitude greater than the adsorbent. The adsorbent is therefore injected downstream of the process filter of a generally known cement production installation and requires an additional filter for separation. However, this filter is very large, requires high investment costs, requires an additional induced draught fan with the energy consumption connected therewith and seldom has the space required for installation in existing plants.
An improvement of this method is described in U.S. Pat. No. 7,141,091 B2 and known under the name TOXECON II. In this instance, the adsorbent is injected between a filter portion which is located upstream and a filter portion which is located downstream. The dust separated in the filter portion located upstream is not contaminated with the adsorbent, whilst in the filter portion located downstream the residual dust is mixed with adsorbent. No additional filter is required in this method. It is disadvantageous that there is still a large disparity between the mass flows of adsorbent with respect to process dust and this process also does not achieve the efficiency of the TOXECON method.
The dust which occurs in both methods must either be disposed of or can be added as a filler to the cement grinding operation. Since it contains carbon, however, this addition is limited. It is particularly serious that, even with very small proportions, a black colour influence of the activated carbon can be seen.
EP 1 386 655 A1 discloses the addition of bromine or bromine-containing compounds. With the bromine, the elemental mercury forms mercury bromide which can be more readily separated than the elemental form. A disadvantage in the production of cement clinker is the addition of a halogen, which forms circuits between the coarse mill or filter and the preheater and is thereby further enriched, and has disadvantageous effects in the production process. The dust which is contaminated with mercury bromide must be flushed out and subsequently processed or disposed of.
There is known from the subsequently published DE 10 2010 004011 B3 a method and an installation for producing cement clinker and for separating nitrogen oxides and mercury from the exhaust gases of the cement production process, the hot preheater gases being reduced in terms of dust content prior to a selective catalytic exhaust gas cleaning operation. During direct operation, the exhaust gas is then supplied via a cooling tower to a dust filter, before the denitrided and dedusted exhaust gas is directed into a mercury washer or an absorber.
EP 0 461 305 B1 describes a method for cleaning the exhaust gases of installations for producing cement clinker, the exhaust gases of a preheating zone being cleaned in a multi-step filter zone by a separation of dust first being carried out in a first filter step and the exhaust gas subsequently flowing through at least one additional filter step, which is constructed as an adsorption step and which contains an adsorbent by means of which NHx compounds, heavy metals, trace elements and/or SO2 contained in the exhaust gas are bound and NOx is at least partially reduced. The exhaust gases of the preheating zone are adjusted prior to the first filter stage to such a filter zone input temperature that, in the first filter step, highly volatile toxic substance elements and toxic substance compounds are separated from the exhaust gases together with the dust.